The present invention relates to a radiation-curable organopolysiloxane-containing composition or, more particularly, to a radiation-curable organopolysiloxane-containing composition capable of being cured by the irradiation with a relatively small dose of radiation to give a cured film having high surface releasability from an adhesive or a sticky substance so as to be useful as a surface-releasing coating composition such as a back-surface treating agent of a pressure-sensitive adhesive tape manufactured in rolls or a surface-treating agent of a release paper sheet used for temporary protection of pressure-sensitive adhesive labels, stickers and the like.
It is known that a surface-releasing coating composition mentioned above is formulated with a silicone or an organopolysiloxane compound as a releasability-imparting ingredient in the composition by virtue of the unique surface property of the organopolysiloxane as cured. Such a surface-releasing coating composition is applied to the surface of a substrate sheet, e.g., paper, and cured by the irradiation with actinic rays such as electron beams, ultraviolet light and the like to give a cured surface-releasing film on the substrate. For example, Japanese Patent Publication No. 52-3190 teaches a silicone-containing surface-releasing coating composition curable by the irradiation with electron beams, of which the organopolysiloxane has (meth)acryloxy, i.e. acryloxy or methacryloxy, groups in a molecule with an organic polymer as the base ingredient. Further, Japanese Patent Publication No. 53-36515 discloses an ultraviolet-curable surface-releasing coating composition comprising an organopolysiloxane of the above mentioned type and a photosensitizer.
A problem in the silicone-containing surface-releasing coating composition of the above mentioned type consists in the difficulty in obtaining a good balance between the radiation-curability of the organopolysiloxane and the surface-releasability to be exhibited by the cured organopolysiloxane. Namely, the content of the (meth)acryloxy groups in the organopolysiloxane molecules should be increased as high as possible when high radiation-curability is desired while an increase in the content of the (meth)acryloxy groups is unavoidably accompanied by a decrease in the surface-releasability of the cured composition as a consequence of the high polarity of the (meth)acryloxy groups in addition to the disadvantage that the storage stability of the composition is decreased.
With an object to solve the above mentioned problem by increasing the surface-releasability, Japanese Patent Publication No. 62-30234 proposes a method in which the silicone-containing surface-releasing coating composition is admixed with a diorganopolysiloxane of a relatively high molecular weight having a linear molecular structure. Such a composition, however, has another problem that, when a pressure-sensitive adhesive tape or sheet temporarily protected with the release paper is peeled off therefrom, the adhesiveness on the adhesive surface is remarkably decreased, especially, when the coating layer has been cured with a small irradiation dose of electron beams.
On the other hand, Japanese Patent Publication No. 57-57096 proposes, with an object to improve both of the radiation-curability and adhesiveness retention, a radiation-curable silicone-containing surface-releasing coating composition comprising a (meth)acryloxy group-containing organopolysiloxane and a (meth)acrylate ester of a polyfunctional alcoholic compound or a low molecular weight organopolysiloxane having (meth)acryloxy groups. Although this composition is satisfactory in respect of the radiation-curability of adhesiveness retention, a problem left therein is that the surface releasability of the radiation-cured layer of the composition is not quite high with a relatively large resistance against peeling of a release paper sheet prepared with the composition from an adhesive surface.